This invention relates to a method for processing a Nixe2x80x94Tixe2x80x94Nb based alloy, to such alloys per se, and to articles made from such alloys.
Nixe2x80x94Ti based alloys are known to exhibit shape memory properties, associated with transformations between martensite and austenite phases. These properties include thermally induced changes in configuration in which an article is first deformed from a heat-stable configuration to a heat-unstable configuration. Subsequent exposure to increased temperature results in a change in configuration from the heat-unstable configuration towards the original heat-stable configuration.
They also exhibit enhanced elastic properties compared with materials which do not exhibit martensite-austenite transformations. The superelastic transformation of a shape memory alloy is discussed in xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d, T W Duerig et al, on page 370, Butterworth-Heinemann (1990. Subject matter disclosed in that document is incorporated in this specification by this reference to the document. The transformation is depicted in FIG. 1 of the accompanying drawings. FIG. 2 shows how stress varies with strain during a reversible elastic deformation. It will be seen that, as strain increases, stress increases initially approximately linearly. This behavior is reversible, and corresponds to conventional elastic deformation. Subsequent increases in strain are accompanied by little or no increase in stress, over a limited range of strain to the end of the xe2x80x9cloading plateauxe2x80x9d. The loading plateau stress is defined by the inflection point on the stress/strain graph. Subsequent increases in strain are accompanied by increases in stress. On unloading, there is a decline in stress with reducing strain to the start of the xe2x80x9cunloading plateauxe2x80x9d evidenced by the existence of an inflection point (which is characteristic of the superelastic behaviour with which the present invention is concerned) along which stress changes little with reducing strain. At the end of the unloading plateau, stress reduces with reducing strain. The unloading plateau stress is also defined by the inflection point on the stress/strain graph. Any residual strain after unloading to zero stress is the permanent set of the sample. Characteristics of this deformation, the loading plateau, the unloading plateau, the elastic modulus, the plateau length and the permanent set (defined with respect to a specific total deformation) are established, and are defined in, for example, xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d, on page 376. Typical values for commercially available Nixe2x80x94Ti binary alloys are:
Loading plateau stress 500 MPa
Unloading plateau stress 150-280 MPa
Permanent set (after 6% deformation)  less than 0.5%
Plateau length 6%-8%
Elastic modulus 40-50 GPa
The thermally induced recovery shape memory properties of Nixe2x80x94Tixe2x80x94Nb based alloys have been investigated. It has been found that the characteristic temperatures of the shape transformation of certain Nixe2x80x94Tixe2x80x94Nb based alloys can be modified by appropriate treatment, so that alloys which would normally exist in the austenite phase at ambient temperature can be stored in the martensite phase at room temperature in the deformed configuration from which they will recover when heated. Such alloys are disclosed in EP-A-185452.
The advantageous properties of the Nixe2x80x94Tixe2x80x94Nb based alloys disclosed in EP-A-185452 lie in their ability to respond to a treatment to change temporarily the characteristic temperatures of the thermally induced change in configuration. No consideration has been given to their superelastic properties.
Indeed, the fact that the transformation hysteresis can be expanded in the way referred to above (to make an alloy stable temporarily at ambient temperature in the martensite phase) suggests that the alloys would not be useful as superelastic alloys, it is established that it is preferable in a superelastic alloy for hysteresis to be as small as possible (see for example xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d, T W Duerig et al, page 382, Butterworth-Heinemann (1990)).
The present invention is concerned with the previously unrecognised superelastic behaviour of Nixe2x80x94Tixe2x80x94Nb alloys, having properties which are superior to those of other alloys which exhibit superelastic behaviour, such as Nixe2x80x94Ti binary alloys.
The invention provides a method of processing a Nixe2x80x94Tixe2x80x94Nb based alloy which comprises working an article formed from such an alloy at a temperature which is less than the recrystallisation temperature of the alloy. Recrystallisation of an alloy involves the formation of new, defect-free, low energy grains or crystals, which consume and replace highly worked, high energy grains. It involves the loss of a textured structure introduced by working.
Accordingly, in one aspect, the invention provides a method of processing a Nixe2x80x94Tixe2x80x94Nb based alloy which contains from about 4 to about 14 atomic percent Nb and in which the ratio of atomic percent Ni to atomic percent Ti is from about 0.8 to 1.2, comprising subjecting the alloy to at least about 10% work by a technique which comprises at least one of rolling and drawing, and operations which produce a similar textured crystal structure in the alloy, at a temperature below the recrystallisation temperature of the alloy.
The method of the invention gives rise to beneficial properties in the processed alloy. In particular, the plateau stresses on both loading and unloading are increased significantly compared with conventional binary alloys. Furthermore, the permanent set for a given deformation can in some circumstances be reduced compared with such conventional alloys. These benefits are important. They make it possible for articles to be made which can store relatively larger amounts of elastic energy per unit volume of material. As a corollary, they make it possible to keep small the size of components made using the article. The increased stiffness that is apparent in articles made using the treated alloy is an attractive feature when the articles are used in, for example, eyeglass frames, orthodontic archwires and guidewires for catheters. The method of the invention provides this increased stiffness without an undesirable permanent set, which has accompanied previous attempts to increase stiffness in superelastic shape memory alloy materials, for example by varying the compositions of the alloys.
A further advantage of the alloys of the invention is that the tendency found in some Nixe2x80x94Ti based alloys to revert to an R-phase (a transitional phase between the austenite and martensite phases) is reduced. This reduces the tendency of elastic modulus to be lowered. It is important for certain applications, for example when the alloy is used in a catheter guidewire when the modulus controls the geometric stability of the wire against lateral stresses.
Yet another advantage of the invention is that it provides articles with superelastic properties which are more resistant to corrosion than articles formed from alloys used previously for their superelastic properties. An advantage arising from the corrosion resistance is the compatibility of the materials which makes them suitable for use in medical applications.
Accordingly, the invention provides a Nixe2x80x94Tixe2x80x94Nb based alloy whose superelastic properties are such that (a) the loading plateau on loading at 25xc2x0 C. is at least about 600 MPa, preferably at least about 700 MPa, more preferably at least about 800 MPa, for example at least about 900 MPa, and (b) the permanent set after tensile deformation at 25xc2x0 C. to 6% is less than about 2.5%, preferably less than about 1.5%, more preferably less than about 1.0%.
Preferably, the work is imparted to the alloy by a technique which comprises at least one of rolling and drawing, and similar operations which produce a textured crystal structure. Surprisingly, it has been found that the beneficial properties which result from the method of the invention arise from working by these techniques. By working the alloy using rolling or drawing (including die-less drawing) or a similar techniques, it has been found to be possible to produce an alloy which displays good ductility and good strength. Other working techniques can produce alloys with one of these properties, but not both.
Particularly preferred working techniques include rod and wire drawing.